1. Field of the Invention
The present invention relates in general to bumper constructions for automotive vehicles and more particularly to improvements in a bumper construction wherein the end sections of a plastic bumper are mounted to a vehicle body by means of a pair of bumper stays.
2. Description of the Prior Art
FIGS. 1 and 2 show a prior art rear bumper construction wherein a plastic bumper 10 is mounted at both end sections thereof to a vehicle body 12 by means of a pair of bumper stays 14. The bumper 10 is made of a plastic material such as polypropylene and has a C-shaped cross section as shown in FIG. 2. A reinforcement member 16 is made of metal and has a cross section similar to the bumper 10. The reinforcement member 16 is mounted to the inner surface of the bumper 10 and is secured thereto by means of bolts 18 and nuts 20. This reinforcement member 16 increases the rigidity of the bumper. In FIG. 2, the vehicle body 12 is shown to comprise, at the rear end thereof and adjacent the bumper 10, a body panel including a rear upper panel member 22, a rear lower panel member 24 and a rear floor panel member 26. The bumper stays 14, however, are mounted to the rear side members (not shown) of the vehicle body 12, respectively.
FIG. 3 sectionally shows a plastic rear bumper 28 used in another prior art rear bumper construction. This bumper 28 has sufficient structural rigidity even though it does not have a metallic reinforcement member as is used in the bumper construction of FIGS. 1 and 2. To provide this rigidity, the wall of the bumper 28 is formed sufficiently thick and in addition to this, a plurality of large spaced-apart ribs 30 are provided along the inner surface of the bumper 28.
The prior art bumper construction shown in FIGS. 1 and 2 has several disadvantages. It is relatively heavy due to the metallic reinforcement member 16. Also, if a permanent deformation of the reinforcement member 16 occurs, the bumper 10 is caused to have a distorted portion or portions even if the bumper itself is not substantially damaged.
The prior art bumper construction shown in FIG. 3 also has several disadvantages. The bumper 28 requires a large quantity of a plastic material and therefore requires a long moulding cycle. This causes high material and manufacturing costs. Furthermore, since the bumper 28 and the ribs 30 are not uniform in thickness and cool and shrink at different rates and amounts, the outer surface of the bumper 28 often undulates slightly. In order to overcome this surface defect and to improve the appearance of the bumper 28, it is necessary to process the outer surface of the bumper by graining. This inevitably results in further increasing the manufacturing cost of the bumper.